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GCSE Week 09 Inheritance
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Inheritance...
GCSE Biology Week 9
This week: InheritanceB11.21GenesexistinalternativeformsB11.22KeytermsB11.23MonohybridGeneticDiagram/PunnettsquaresandfamilypedigreesB11.24CalculateandanalyseoutcomesfrommonohybridcrossesB11.25SymptomsofsicklecelldiseaseandcysticfibrosisB11.26PedigreeanalysisscreeningforgeneticdisordersB31.17ThesexofapersoniscontrolledbyonepairofchromosomesB31.18Howthesexofoffspringisdeterminedatfertilization(geneticdiagram)B31.19Howsex-linkedgeneticdisordersareinherited
How do cells divide to become
anything at all?
Somaticcellsaredescribedas................(fromtheGreekmeaning‘double’).…youwillseethiswrittenas“2n”
DIPLOID
Through a process called… Mitosis!
RememberGCSEBiologyWeek4:CellDivision?
Some cells in the human body are not diploid... Gametes contain only 1 copy of each gene as they have only 1 set of chromosomes.
These cells are .............. and are produced by a
special type of cell division called ............Haploid
Meiosis
A male and Female gamete join together at fertilisation...
This 1 cell then divides
by
forming a. zygote
Mitosisto produce a complete
new organism...
Youreceivedonechromatidfromyourfather.
Andonefromchromatid yourmother!
Remember…thatinalllivingthings,characteristicsarepassedoninthe
chromosomesthatoffspringinheritfromtheirparents.
GENEforEyecolour
GENEforEyecolour
Eachchromosomemayhaveadifferentversion ofagene.
GENE/Allele forBlueEyecolour
GENE/Allele forBrownEyecolour
Differentversionsofagene,thatcodefor
differentversionsofacharacteristic,arecalled
ALLELES.
Homozygousindividualsaretruebreeding.Thismeansthattheywillalwaysproducethesamephenotypeofoffspringbecausetheyarenot‘hiding’arecessiveallele.
Ingenetics,differentallelesareoftenrepresentedbyletters,suchasAa.ThecapitalletterstandsfortheDOMINANT allele‘A’;whereas‘a’wouldbetherecessive allele.
Monohybrid inheritance: how single genes are passed on... Medal and his peas…
Monohybrid (single- gene)inheritanceconcernstheinheritanceofdifferentalleles(usually2)ofasinglegene.(e.g.thegeneforheight)
...LikeMendel,we’llstartwiththepeaplants,whichhaveeasilyobservablefeaturesthatarecontrolledbyasinglegene.
e.g.Peaplantshaveonegeneforheight.Theheightgenehas2alleles:
T(TALL)
t(dwarf)
Peaplantsarediploid andsohave2alleles (forheight).Thereforethereare3possiblegenotypes
TT = homozygous dominant (homo = same)
(so homozygous for T)
Tt = Heterozygous(hetero = different)
tt = homozygous recessive (homo = same)
(so homozygous for t)
ConsiderwhathappenswhenahomozygousTallplant(T)iscrossedwithahomozygousdwarf(t)plant.
T
tHomozygousDwarfPlant(tt)
Homozygous TallPlant(TT)
TT
tt
tt
TTT
t
AllgametesfromtheTallplantcontainaTallele,andallthosefromadwarfplantcontainatallele.Thesecombineatfertilisationtogive
offspringallwiththegenotypeTt(thegenes/allelesthatdetermineanorganismsphenotype)
However,althoughtheylookidentical inrespecttotheirTallparentplant,theyareverydifferentinoneveryimportantrespect:theyareHeterozygousandNOT
homozygous.
If2oftheseheterozygousplantsarecrossed,halfofthegametesfromeachparentareTandhalfaret,givingus4
possiblegenotypesinthesecondgeneration(F2)
T
tHeterozygous(Tall)Plant(Tt)
Heterozygous(Tall)Plant(Tt)
TT
tt
TT
ttt
T
The1st 3giveTallplants,the4th givesadwarf,so75%Tall,25%dwarf.
25%=TT50%=Tt25%=tt
T
tHeterozygous(Tall)Plant(Tt)
Heterozygous(Tall)Plant(Tt)
TT
tt
TT
ttt
T
B11.22Keyterms:Youhavetobeabletodefine thefollowing
terms…
HomozygousRecessive
Monohybrid inheritance in humans:
Clear-cutexamplesofmonohybridinheritanceinhumansarerelativelyrare,andofteninvolvegeneticdiseasewherepeopleinherit1ormorefaultyalleles.
Geneticdiseasesareoftenrecessive;thisisbecausefaultyallelesthatfailtomakeanimportantproteincanbe
maskedbynormalonesthatfunctionproperly.
i.e.RecessiveallelesaremaskedbyDominantAlleles.
Incontrastsomegeneticdisease,e.g.Huntington'sdisease,arecausedbyDOMINANTALLELES.
Theallelesconcernedcodeforaproductthatactivelycausesdamage;symptomsarenotduetoanallelenotdoingitsjob.Suchallelesaredominant becausethepresenceofanormalallelecannotmaskthesymptoms.
Someexamplesofmonohybridinheritanceinhumans:Traits Features
Dominant traits
Huntington'sdisease Symptoms in30’sand40’s,degenerative.
Freckles Freckles
Dimpleinchin Dimpleinchin
Recessivetraits
Sickle cellanaemia(B11.25Symptomsofsicklecelldiseaseandcysticfibrosis)
Haemoglobinpolymerises,distortingredbloodcellsintosickleshape – blockageofcirculatorysystemandanaemia.
Albinism Inabilitytomakepigment melanin.
Lactoseintolerance Inabilitytobreakdownthedisaccharidelactoseintoglucoseandgalactose.Leadstovomiting,diarrhoea, flatulence.
Cystic fibrosis(B11.25Symptomsofsicklecelldiseaseandcysticfibrosis)
Excessivemucous production,especiallyinthelungsandpancreas.Breathinganddigestionarealsoaffectedandsuffersareverysusceptibletolunginfections.
(B11.26Pedigreeanalysisscreeningforgeneticdisorders)
P Q
R S
Affected male
Unaffected male
Affected female
Unaffected female
Key
e.g.Huntington'sdisease:arareinheriteddisorderofthenervoussystem.CausedbyadominantalleleH.Therecessivealleleofthisgeneisrepresentedbyh.
ThediagramshowstheinheritanceofHuntington’sdiseaseinafamily.
P Q
R S
Affected male
Unaffected male
Affected female
Unaffected female
Key
UseageneticdiagramtoshowtheinheritanceoftheHuntington'sDiseaseallelebythechildrenofparentsPandQ.
hh
Hh
Hh
hh
hh Hh
h h
h hh h
H h
hH hH hh hh
Father: Mother:
Genotype:
Alleles: (ineggandsperm)
Poss.CombinationsofAlleles
Phenotype: normalsuffer
meiosis
normalsuffer50%Chance
B11.24Calculateandanalyseoutcomesfrommonohybridcrosses
Q)ExplainwhynoneofthechildrenofRandSinheritedHuntington’sdisease.
P Q
R S
Affected male
Unaffected male
Affected female
Unaffected female
Key
hh
Hh
Hh
hhhh
hhhh hh hh
A)Bothparentsareunaffected,Thustheydon'thavethedisease.TheGenotypeofparentsis...hhhomozygousrecessive(orneitherparenthasHdominantgene/allele).
Whatifbothparentsareheterozygous?OrganiseyourresultsintoaPunnetsquare:
H
hHeterozygousMother(Hh)
HeterozygousFather(Hh)
HH
hh
HH
hhh
H
25%=HH50%=Hh25%=hh
B11.24Calculateandanalyseoutcomesfrommonohybridcrosses
3:1
Ff Ff
F f
F Ff f
F f
FF fF Ff ff
Father: Mother:
Genotype:
Alleles: (ineggandsperm)
Poss.CombinationsofAlleles
Phenotype: normal carrier carrier suffer
meiosis
B11.23MonohybridGeneticDiagramPunnettsquaresandfamilypedigrees…Cysticfibrosisisarecessivedisorder,socross2carrierswithgenotypesFf
OrganiseyourresultsintoaPunnetsquare:
F
fHeterozygousMother(Ff)
HeterozygousFather(Ff)
FF
ff
FF
fff
F
25%=FF50%=Ff25%=ff
3:1
Meiosis: The production of Gametes (sex cells)
Remember sperm and egg (ova) are Hapliod – having just “n” half the number of
chromosomes.
Meiosis is a special kind of cell division in which there are 2 successive divisions
that result in the production of gametes –
the sex cells!
Meiosis: The production of Gametes (sex cells)
B3 1.17 The sex of a person is controlled by one pair of chromosomes.
B3 1.18 How the sex of offspring is determined at fertilization (genetic diagram)
XY XX
X Y
X XY Y
X X
XX XY XX XY
Father: Mother:
Genotype:
Gametes
Poss.Combinations
Phenotype:i.e.Sex Female Male Female Male
meiosis
B3 1.17 The sex of a person is controlled by one pair of chromosomes.
B3 1.18 How the sex of offspring is determined at fertilization (genetic diagram)
OrganiseyourresultsintoaPunnetsquare:
X
X
Female=XX
Male=XYXX
XX
XX
YYY
X
50%=XX50%=XY
1:1
Sex linked Genetic disorders…
B31.19Howsex-linkedgeneticdisordersareinherited
Sex linked Genetic disorders…
Somegeneticcharacteristicsaresexlinked– meaningtheyarecarriedononeoftheSexChromomsomes.(XorY)
Colour Blindness is inherited, and is caused
by a faulty allele on the X chromosome…
Since the Y chromosome is smaller than the X chromosome
it carries fewer sex-linked genetic disorders…
So, colour blindness, for
example is much more common in
men than in women – because men only
require 1 (recessive) faulty allele – whereas women need 2. XnY
XnXnThis genotype is
rare…
This genotype is more common…
XNY XNXn
XN Y
XN XNY Y
XN Xn
XNXN XNY XNXn XnY
NormalMale:
ParentsGenotype:
GenotypeofGametes
Poss.Combinations
Offspring'sPhenotype:i.e.colourblind/normalorcarrier
NormalFemale
NormalMale
NormalFemaleCarrier
ColourBlindMale
meiosis
NormalFemale(carrier)ParentsPhenotypeN=AlleleforNormal
colourvision
n=Alleleforcolourblindness– (Faultyallele)
OrganiseyourresultsintoaPunnetsquare:
XN
XN
Female=XNXn
Male=XNY
XNXN
XNXN
XnXn
YYY
Xn2:1:1
3:1
Theratiois3:1(normal:Colourblind)
butcanalsoberepresentedas2:1:1(Normal:Carrier:
ColourBlind)
Sex linked Genetic disorders…
Haemophilia(adiseasewhereblooddoesn'tclotproperly)isageneticdiseasethatisinheritedinexactlythesamewayascolour blindness.
Haemophilia is inherited, and is caused by a faulty
allele on the X chromosome…
Homozygous:Pairofallelesthatproduceacharacteristicarethesame,e.g.Homozygous dominant=HHHomozygous Recessive=hh
Heterozygous:Pairofallelesthatproduceacharacteristic;theallelesaredifferent,e.g.Hh.
Recessive:Anallelethatwillonlybeexpressedwhenbothallelesareofthistypee.g.hh.
Dominant:Anallelethatwillalwaysbeexpressedevenwhenthereisonlyoneoftheseallelespresent, representedbyacapitalletter.e.g.HHorHh.– theH(dominateallele– willbeexpressed)
Gene:SectionofDNAthatcodesforaparticulartraitorcharacteristic.
Allele:Adifferentformofagenethatcodesforadifferentversionofacharacteristic.
Genotype:Adescriptionofthepairofallelespresentforacharacteristic.
Phenotype:Thephysicalexpressionofthealleles.
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